Already searched several forums and big google, didn't find a solution. Therefore I would like to post this question here. Perhaps someone has an idea.

I have several external calendars located on Synology Diskstation Calendar App. The calendars can be accessed perfectly by CalDav protocol. Some calendars are readonly for some users, others have full access.The apps I use:- Android ical import / export together with aCalendar+- Thunderbird 52.5.0, Lightning 5.4.4, on Win8.1- Synology Diskstation DS215+ with DSM6.1, Calendar 2.1.0-0338 (always uptodate)

The Android ical app doesn't have any issues at all. The readonly I cannot manipulate on my Android. The full access calendars I can change whenever I want to. And all synchronizes perfectly with Diskstation Calendar.

With Lightning on the other hand I only sometimes can change events of calendars with full access.All calendars are set to "Offline Support"Somehow it happens that (e.g. just for test)- I changed an event yesterday.- Of course I switch of my PC.- Try to change same event today.- Event seems to be readonly. I can do the changes in the event dialog. But when I hit ok / save the changes will not appear in the calendar.

After some testing I found: When this problem occurs Lightning error log shows an error 405 "method not allowed".

And there is an odd workaround: If I just switch the calendar "Offline Support" to off and on again. The problem disappears until... at some circumstance the problem is there again.And, this issue doen't happen with one external calendar but all CalDav calendars.

The Android app working on same calendars does not have these issues. I can change same event I vainly try on Lightning.

I don't know that platform but have looked very briefly at Synology. I also use Lightning and aCalendar. One difference between aCalendar and Thunderbird is that while aCalendar keeps a local copy (the equivalent of offline mode), it also pushes your changes to the server immediately, whereas Lightning updates at regular intervals.

Could it be that Synology doesn't like the offline mode? That it rejects changes you do locally when you don't send them immediately to the server? So there are two possible solutions you might try.

1. If you don't use Thunderbird in environments without any internet connection, disable "Offline Support", [quit and restart Thunderbird], and see how your calendars behave when you add an event on Thunderbird / Lightning. If you can add events without problems, then you have found the solution.BTW, the only difference between Offline support on vs off is that if you don't have an internet connection and offline support is off, your calendars will be empty.

One worry: I'm not sure whether disabling offline support means that new events are sent to the server immediately or not.

2. If you need Offline support (or if option 1 doesn't work), try the following:– Create a new event– Right-click under the list of calendars and immediately click on "Synchronize now". That way, your events will be synchronized without delay, and maybe that what is needed for their servers.

About 1.:"Offline support" off seems to work. But then, as you write, the calendars would be empty if I take my Laptop on a journey.So, it would be good to have offline support.

Thunderbird also seems to push an event to CalDav as I create or change it. I can see it with very little delay (<< 1min) on Synology Calendar App.

About 2.:Strange: Despite of not being able to change events of a calendar I am able to add an event to same calendar.This new event I can change (although not being able to change the others) - until e.g. next day - then this new event is blocked as well.

Even more confusing: If I change an event in Synology Calendar App (or Android) then this one event seems to unblock and I can change it with Lighting as well (until it gets blocked next day).So it seems that although the blocking hits a whole calendar it effects every single event.You think it could be a problem of Synology Calendar ? I'll try to retrieve some log output.

You also are using aCalendar and Lightning ? How do you synchronize them ?